The NTSB's declaration is just a recommendation, but note that it previously recommended that .08 be the cut-off -- a standard that happens to apply to laws in all 50 states. The NTSB says it would want lawmakers to approve "incentive grants" to states that adopt the new .05 level, which is the same reason all 50 states now have limits of .08.

(Yes, Arizona law still has a law against driving while impaired to the "slightest degree," which doesn't require a blood-alcohol level of .08.)

The NTSB's safety report, released today, says studies showed that some of the countries that have already lowered their legal limit from .08 to .05 saw fewer fatal crashes.

"The NTSB concludes that BAC levels higher than 0.05 are viewed by respected traffic safety and public health organizations around the world as posing unacceptable risk for driving, and more than 100 countries have already established per se BAC limits at or below 0.05," the report says.

So, why is this exact number the new level that would put Arizona drivers in jail in many cases, and pay astronomical fines, while possibly having to find a new way to get to work (if you still have that job after the jail time)?

That's hard to say, since the NTSB itself concludes that "BAC levels as low as 0.01 have been associated with driving-related performance impairment, and BAC levels as low as 0.05 have been associated with significantly increased risk of fatal crashes."

A chart in that section (page 21) shows that the "relative crash risk" at a blood-alcohol level of .06 is hardly higher than .05. Why not .06? Who knows.

Then again, it might not matter that much. If you look at Arizona's stats from the Governor's Office of Highway Safey for DUI enforcement last year, the average blood-alcohol level was higher than .15 in more than 30,000 DUI arrests.